The benefits of using water-source heat pumps

Water-source heat pumps prove to be more energy-efficient than alternative systems for commercial buildings.

Naveen Halbhavi, PE, ClimateMaster Inc., Oklahoma City

09/07/2016

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Learning Objectives:

Analyze water-source heat pumps and compare them to alternative systems.

Examine how water-source heat pumps can reduce building energy consumption.

Implement a water-source heat pump system to meet simultaneous heating and cooling demand.

The first commercial energy code was promulgated in 1975 with the ASHRAE Standard 90-1975: Energy Conservation in New Building Design. Since then, subsequent efforts have led to more stringent energy efficiency standards, as shown in Figure 1.

A complete and uniform adoption of these energy codes would have resulted in approximately a 50% reduction in normalized energy use between 1975 and 2012. In reality, total energy consumption per square foot in commercial buildings has decreased from 114 kBtu/sq ft in 1979 to 79.9 kBtu/sq ft in 2012—a 30% decrease.

While this is a significant achievement, adoption and enforcement of standards by different states has not been uniform, and buildings continue to account for a large percentage of energy consumption in the U.S. According to the U.S. Energy Information Administration, commercial buildings consumed 7 quadrillion Btus of energy. Further, HVAC accounts for 44% of the commercial building energy demand, as shown in Figure 2. This includes space heating, ventilation, and cooling, but excludes refrigeration.

Commercial heat pumps

A heat pump is a refrigeration circuit that can cool spaces during warm weather and heat spaces during cool weather. With a heat pump, you can cool or heat a space by only using electricity. By not burning fuel for heating, as in a traditional central furnace, a flammability risk is eliminated.

Commercially available heat pumps can be categorized into two broad types:

An air-source or air-cooled heat pump

A water-source heat pump (WSHP).

An air-source or air-cooled heat pump is a type of heat pump that operates by rejecting heat to outside air during the summer or by absorbing heat from outside air during the winter. A WSHP is a type of heat pump that operates by rejecting heat to a water-pipe system (or water loop) during the summer or by absorbing heat from the same water loop during the winter. If multiple units of WSHPs are installed, they can all be serviced by a common water-loop system (or header).

Advantages of water-source heat pumps

For WSHPs, since the heat is transferred via a heat exchanger into a pipe that is carrying water, the operation is quieter and the system footprint is smaller since water is more efficient at carrying away heat than air. In an air-source system, the limiting heat-transfer coefficient is on the air side and typical forced convection air-side heat-transfer coefficients is in the range of 25 to 250 W/m2 K. In contrast, the forced convection heat-transfer coefficient on the water side is between 50 to 20,000 W/m2 K. This makes WSHP equipment more efficient and smaller in size than air-source heat pumps.

Traditional air source units can require each air handling unit to have a separate condensing unit. For a large, multi-unit system, which is common in a commercial building, multiple condensing units would be needed that are not only noisy but also present a challenge to install since they require a lot of free space. With a multi-unit WSHP installation, heat exchange can be accomplished with a single, central evaporative cooling tower or dry cooler located on the ground or the rooftop. The WSHP units can be placed in dropped ceilings or hidden away from occupied spaces in mechanical rooms or utility closets. Placing the units in ceilings, near to the point of use, also results in less ductwork and less fan-energy consumption. Fan-energy consumption can be among the largest energy components of an HVAC system, and a good overall system design will attempt to minimize it.

WSHPs also offer some of the highest efficiencies in the HVAC industry. ASHRAE sets the minimum efficiency requirements for WSHPs to be higher than traditional air-cooled heat pumps and VRF systems. Tables 1 and 2 show efficiency values for the most directly comparable units and is derived from ASHRAE 90.1-2013: Energy Standard for Buildings Except Low-Rise Residential Buildings. This comparison shows that WSHPs meet the highest minimum energy efficiency ratio (EER) and coefficient of performance (COP) requirements.

WSHPs also are more efficient at heating when compared with packaged furnace air conditioners. In a furnace unit, the maximum efficiency for heating by burning natural gas is about 95% (for a COP of 0.95); electrical heat is 100% (COP = 1.0). With a water-source heat pump in heating mode, not only is the thermal energy from the water loop being extracted and used, but also the heat of compression in the refrigerant circuit is captured and used as a source of heating. Due to this capability of extracting heat from a heat source (i.e., the water loop) and using the heat of compression, the WSHP can easily provide 4 to 6 units of heating for every unit of energy consumed. Clearly, this is a more efficient system.